The invention is an improvement over the state of the art represented for example, by Swiss Patent Specification No. 631,950. Apparatuses of this type are used increasingly for sterilizing industrial, drinking or swimming-pool water, or also beverages, essences, concentrates or oils, provided that the last-mentioned liquids have sufficient permeability to the ultraviolet rays (that is to say, ultraviolet rays can pass through them). Because the discharge tube of the ultraviolet lamp possessed a tubular extension for adjusting the mercury vapor pressure, in the apparatuses used hitherto the space in which the ultraviolet lamp was fastened was essentially divided into three chambers separated from one another in thermal terms: a bottom chamber in which the tubular extension of the ultraviolet lamp was located, a middle chamber in which there was the discharge space of the lamp and round which flowed the liquid to be sterilized, and an upper part in which the electrode bulbs were accommodated and which was cooled by cooling fins. The three-chamber division therefore appeared to be necessary in order to ensure, on the one hand, that the operating temperature of the discharge zone (as a rule, .gtoreq.300.degree. C.) did not overheat the tubular extension which, as the coldest point of the system (as a rule, .ltoreq.65.degree. C.), is responsible for the mercury vapor pressure and consequently also for the radiation intensity, and, on the other hand, that the upper space was also not overheated by the discharge zone, because this space accessible from outside was not allowed to become too hot for safety reasons.
So that the ultraviolet lamp could be ignited at all, the entire portion round the tubular extension had to be preheated via a heating resistor before ignition. This heating was then cut out again when the anode current was switched on. During operation, the temperature of the tubular extension was determined by the backheating of the discharge zone and the temperature of the ambient air. This in turn fluctuated with the water temperature. Depending on the season, the temperature of the tubular extension therefore had to be corrected by suitable intermediate rings, by means of which the tubular extension was embedded to a greater or lesser extent in the bottom chamber.
The disadvantage of the previous apparatuses was that, when the tubular extension was heated to ignition, the middle part, and consequently the discharge zone, remained cold. The mercury was therefore able to condense again there, thus making ignition more difficult. On the other hand, during operation, the high temperature of the discharge zone in the middle chamber had an extremely detrimental effect on the service life of the ultraviolet lamp.
In Swiss Patent Specification No. 631,950 mentioned in the introduction, a low-pressure high-current mercury-vapor lamp equipped with a tubular extension arranged on the discharge tube is used as a radiation source. So that the temperature of the tubular extension can be regulated specifically even during the operation of the ultraviolet lamp, and to ensure a longer service life of the ultraviolet lamp, an airstream circulating through the apparatus is used to cool the ultraviolet lamp. This airstream is guided in such a way that the air heated at the discharge tube flows past the electrode bulbs of the ultraviolet lamp and into a connecting pipe to the outer wall of the line pipe, through which the liquid to be sterilized flows. The air cooled on the outer wall of the line pipe is then delivered to the ultraviolet lamp once again and must have a temperature which is less than the operating temperature of the tubular extension. The setting of the predetermined temperature is regulated via the sensor by means of a heater.
To reduce the power consumption for heating the tubular extension, Swiss Patent Specification No. 636,479 proproposes to divide the cooling stream into two parts:
One part stream is conveyed directly to the discharge tube of the ultraviolet lamp; the other part stream, which is smaller in comparison with the first, is heated by the heater and delivered to the tubular extension.
Apparatuses according to the Swiss Patent Specifications mentioned have proved outstanding in the past in many uses and have made a substantial contribution to the operating reliability and to a longer service life and therefore greater availability of the installations equipped with these lamps.